Best modeling workflow for painting textures
If you are creating materials for others, it is a common convention to test them on a shader ball. See this question: Cycles advanced shader ball?

However, if you already have a mesh, it is best to test the material on it. The reason is that only the simplest materials are 100% mesh-independent, the rest have to be adjusted to the mesh, for example the depth of subsurface scattering depends on the scale of the mesh.